Contact pressure sensors are used to measure contact pressures between two surfaces. Knowledge of the pressure that exists between two surfaces may be beneficial for many applications. One application exists between the flanges and gasket surfaces in a pipeline. Uneven tightening of bolts may distort the joints and cause leakage. Furthermore, due to various forces acting on the joint, the relaxation of joints etc., it is important to have real time information of the pressure distribution at the mating surfaces of the joints.
For epitaxial film materials such as AlGaAs, the material of substrate wafer used is Gallium Arsenide (GaAs). Contact pressure sensors are based on solid state physics of the pressure sensitivity of semi-conductor elements from columns IIIB and VB of the Mendeleev table. Different column elements are combined, and are grown epitaxially (lattice-matched) on a single substrate. Once contact pressure is applied to the material a difference in resistance can be measured. Additionally, linearity response can be detected to pressure variations by direct contact instead of by hydrostatic means.
Contact pressure sensors to date have typically been made using piezo-resistivity property of Aluminum Gallium Arsenide (AlGaAs) material, an epitaxial film. Of course, the wafer substrate that the epitaxial film is grown has the same lattice structure as the epitaxial film.
An example of a contact pressure sensor using materials with piezo-resistive properties is a meander resistor which is formed by the conventional fabrication processes, like photolithography, etching and metallization.
A type of solid state pressure sensor of AlGaAs is disclosed in U.S. Pat. No. 4,965,697, however, the sensor is based on hydrostatic pressure principles and limited to configurations employing hydrostatic applications.
Various attempts have been performed to overcome the problem of limited pressure values achievable in contact mode. These include (a) thinning of GaAs substrate, (b) enclosing chips in a silicon gel-material to shield the devices and (c) reducing the chip size. However the problem of the chips breaking under high pressures still occurs. Thus, a need exists for a contact pressure sensor to monitor pressure distribution between two surfaces that can withstand direct contact pressures.